Re: Is Recovery actually paused?

On Thu, Feb 4, 2021 at 10:28 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> Please find an updated patch which addresses these comments.

Thanks for the patch. I tested the new function pg_get_wal_replay_pause_state:

postgres=# select pg_get_wal_replay_pause_state();
pg_get_wal_replay_pause_state
-------------------------------
not paused
postgres=# select pg_wal_replay_pause();
pg_wal_replay_pause
---------------------

(1 row)

I can also see the "pause requested" state after I put a gdb
breakpoint in WaitForWALToBecomeAvailable in the standby startup
process .

postgres=# select pg_get_wal_replay_pause_state();
pg_get_wal_replay_pause_state
-------------------------------
pause requested
(1 row)

postgres=# select pg_get_wal_replay_pause_state();
pg_get_wal_replay_pause_state
-------------------------------
paused
(1 row)

Mostly, the v10 patch looks good to me, except below minor comments:

1) A typo in commit message - "just check" --> "just checks"

2) How about
+ Returns recovery pause state. The return values are <literal>not paused
instead of
+ Returns recovery pause state, the return values are <literal>not paused

3) I think it is 'get wal replay pause state', instead of { oid =>
'1137', descr => 'get wal replay is pause state',

4) can we just do this
/*
* If recovery pause is requested then set it paused. While we are in
* the loop, user might resume and pause again so set this every time.
*/
if (((volatile XLogCtlData *) XLogCtl)->recoveryPauseState ==
RECOVERY_PAUSE_REQUESTED)
SetRecoveryPause(RECOVERY_PAUSED);
instead of
/*
* If recovery pause is requested then set it paused. While we are in
* the loop, user might resume and pause again so set this every time.
*/
SpinLockAcquire(&XLogCtl->info_lck);
if (XLogCtl->recoveryPauseState == RECOVERY_PAUSE_REQUESTED)
XLogCtl->recoveryPauseState = RECOVERY_PAUSED;
SpinLockRelease(&XLogCtl->info_lck);

I think it's okay, since we take a spinlock anyways in
GetRecoveryPauseState(). See the below comment and also a relevant
commit 6ba4ecbf477e0b25dd7bde1b0c4e07fc2da19348 on why it's not
necessary taking spinlock always:
/*
* Pause WAL replay, if requested by a hot-standby session via
* SetRecoveryPause().
*
* Note that we intentionally don't take the info_lck spinlock
* here. We might therefore read a slightly stale value of
* the recoveryPause flag, but it can't be very stale (no
* worse than the last spinlock we did acquire). Since a
* pause request is a pretty asynchronous thing anyway,
* possibly responding to it one WAL record later than we
* otherwise would is a minor issue, so it doesn't seem worth
* adding another spinlock cycle to prevent that.
*/

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com